Clad metals are composite materials made by mill rolling or explosion forming a thin sheet of a metal that has desired properties, over and metallurgically bonded at the interface to a base plate of carbon steel or other backing metals. The composite material has the structural strength of the backing metal and the corrosion resistance, abrasion resistance, or other useful properties on the clad face. In the weld-joining of the clad metal, it is essential to preserve the properties of the cladding at the weld joint. Otherwise, the desired properties of the clad surface would not be continuous. The properties of the backing metal must also be maintained. It is, therefore, necessary to use special weld procedures. In the description herein the term "cladding metal" refers to the metal layer used for its surface properties in the clad metal seal composite, while "backing" metal refers to the layer generally used for its structural properties, or for economy.
In one welding procedure, the structural backing metal such as steel, is fusion welded using steel for the weld, and the cladding metal sheet portion is welded, using the cladding metal material for a seal closure. In such a procedure, the cladding metal seal closure will be contaminated to a certain extent with the backing metal, although there is no good test to determine the degree of contamination, and this may impair its chemical resistance or other desired properties. This is particularly true if the welding of the cladding metal requires welding temperatures well above the melting point of the backing metal. In view of this problem, a cover strip of the cladding metal is often placed over the cladding metal closure weld, and welded at its edges to the cladding metal sheet. This type of joint also has limitations in that the cover strip is subject to fatigue fracture, and there is no satisfactory test to evaluate the integrity of the cover strip attachment weld, which welds often prove inadequate in service. Also the fusion welding procedure for attaching the cover strip will heat up the cladding metal surface layer and the backing layer behind it; if the cladding metal is of much higher melting point, the backing metal will be affected sufficiently to cause contamination of the cladding metal.
In view of the problems encountered in heating the backing metal while welding a surface cladding metal, an interlayer of a heat dissipating metal may be used. The use of such a layer is most common for composites in which the cladding metal is of very high melting point, such as tantalum or nobium, while the structural backing metal has a much lower melting point. In closure joints for such a composite, the surface and interlayers can be machined back from the edges for the pieces to be joined, and the backing metal of the pieces is joined by a fusion structural weld. A filler strip of the clad metal material is then placed in the recess over the structural weld, with the edges of the strip abutting the edges of surface and interlayers of the pieces. A cover strip of the cladding metal is then placed over the filler strip and extending beyond it to be attached to the surface of the cladding metal by welds. Again, there is a problem of the integrity of the welds attaching the cover strips. There is no good inspection method, and failures in some types of service are frequent and expensive. Also, cover strips of some metals, e.g. tantalum and niobium, are often relatively thin and flexible and tend to move up and down with changes in temperature and pressure during use. Such movement can result in fatigue fracture of the attachment weld.